1. Field of Invention
The present invention relates to a capacitor array unit, a capacitor array unit controller, an oscillation circuit, a voltage-controlled oscillator, an oscillation frequency adjusting system, and an oscillation frequency adjusting method. In particular, the present invention pertains to a capacitor array unit, a capacitor array unit controller, an oscillation circuit, a voltage-controlled oscillator, an oscillation frequency adjusting system, and an oscillation frequency adjusting method that are used in and for a voltage-controlled piezoelectric oscillation circuit.
2. Description of Related Art
[1] First Conventional Configuration
FIG. 31 is a circuit diagram showing the configuration of a first type of conventional voltage-controlled piezoelectric oscillation circuit. The voltage-controlled piezoelectric oscillation circuit, generally denoted by 100, has the following parts or components: an output terminal OUT through which an oscillation signal SOSC of an oscillation frequency fOSC is output; a frequency-control terminal VC which receives a control voltage VC for varying the oscillation frequency fOSC; an input resistor Ri having one end connected to the frequency control terminal VC and intended to coarsely couple an oscillation frequency control circuit (not shown) that is connected by a user to the frequency control terminal VC while eliminating undesirable effect of the oscillation frequency control circuit on the oscillating stage; a piezoelectric resonator X connected to the other end of the input resistor Ri; a variable-capacitance diode (referred to as "varicap", hereinafter) Cv having a cathode connected to a node between the input resistor Ri and the piezoelectric resonator X; and a trimmer capacitor (referred to also as a "trimmer", hereinafter) CT having one end connected to the anode of the varicap Cv and the other end connected to a lower-potential power supply GND.
The voltage-controlled piezoelectric oscillation circuit 100 also has the following parts or components: a bias resistor RX having one end connected to a node between the varicap Cv and the trimmer CT and the other end connected to the lower-potential power supply GND; a first bias resistor R1 having one end connected to a higher-potential power supply VCC and the other end connected to the other end of the piezoelectric resonator X; a second bias resistor R2 having one end connected to a node between the piezoelectric resonator X and the first bias resistor R1 and the other end connected to the lower-potential power supply GND; an NPN transistor Q1 having a base connected to a node between the piezoelectric resonator X and the bias resistor R1; and a collector resistor Rc having one end connected to the higher-potential power supply VCC and the other end connected to the collector of the NPN transistor Q1.
The voltage-controlled piezoelectric oscillation circuit 100 further has the following parts or components: a DC cut-off capacitor CCO having one end connected to a node between the collector resistor Rc and the NPN transistor Q1 and the other end connected to the output terminal OUT, intended to remove DC component of the oscillation signal SOSC; an emitter resistor Re having one end connected to the emitter of the NPN transistor Q1 and the other end connected to the lower-potential power supply GND; a first oscillation capacitor CO1 having one end connected to a node between the base of the NPN transistor Q1 and the piezoelectric resonator X and the other end connected to a node between the emitter of the NPN transistor Q1 and the emitter resistor Re; and a second oscillation capacitor CO2 having one end connected to a node between the emitter of the NPN transistor Q1 and the emitter resistor Re and the other end connected to the lower-potential power supply GND.
[2] Second Conventional Configuration
FIG. 32 shows a second type of conventional voltage-controlled piezoelectric oscillation circuit. In this Figure, reference numerals that are the same as those appearing in FIG. 1 denote the same parts or components as those of the first type of known arrangement shown in FIG. 31.
The second type of conventional voltage-controlled piezoelectric oscillation circuit 200 differs from the voltage-controlled piezoelectric oscillation circuit 100 of he first type in that the bias resistor RX is omitted, that the anode of the varicap Cv is connected to the lower-potential power supply GND and that the trimmer CT is connected to the varicap Cv in parallel therewith.
[3] Role of the Trimmer CT in the First and Second Conventional Arrangements
Actual oscillation center frequency f0' that appears when a calibrated control voltage is supplied deviates from the target center frequency f0 due to, for example, variation in the characteristics of the components such as the piezoelectric resonator X and the varicap CV. The trimmer CT is intended to compensate for such a deviation, thereby controlling the actual frequency f0' in conformity with the target center frequency f0.
Thus, the actual frequency f0' is regulated to the target center frequency f0, provided that the capacitance value of the trimmer CT is adequately controlled.
In other words, the role of the trimmer CT is to correct deviation of the center frequency that appears due to variation of characteristics of components of the voltage-controlled piezoelectric oscillation circuit, such that the oscillation frequency coincides with the target oscillation frequency when a calibrated control voltage is applied to the oscillation circuit. In general, therefore, an adjustment of the trimmer CT is executed as the final step of the process for fabricating the voltage-controlled piezoelectric oscillation circuit, prior to the shipping of the same.
In an example, the capacitance value of the trimmer CT is adjusted such that a center frequency f0 of 13.0 (MHz) is obtained when a control voltage Vc of 2.5 (V) is applied.
The trimmer CT inherently is not intended to encourage the user to trim the oscillation frequency through adjustment thereof. However, when the voltage-controlled oscillation circuit is actually mounted on, for example, a printed circuit board, the oscillation center frequency f0' may deviate from the target center frequency f0 due to, for example, a thermal stress. In such a case, the trimmer (CT can effectively be used to permit adjustment for the purpose of eliminating the deviation of the actual oscillation frequency f0 from the target oscillation center frequency f0.
[4] Role of the Varicap Cv in the First and Second Conventional Arrangements
The oscillation frequency fOSC is controllable by varying the capacitance CCv of the varicap Cv. The capacitance CCv in turn is controlled by varying the level of the control voltage Vc supplied to the frequency control terminal VC. For instance, in the aforesaid example, the oscillation frequency fOSC is controlled in the range of: EQU fOSC=13.0 (MHz).+-.100 (ppm)
when the frequency control terminal VC is supplied with a control voltage Vc as follows. EQU Vc=2.5.+-.2.0 (V) PA1 where CP represents the equivalent parallel capacitance of the piezoelectric resonator X, while CS is the equivalent series capacitance of the same.
[5] Principle of Operation of the Voltage-controlled Piezoelectric Oscillation Circuit
A description will now be given of the principle of operation of the voltage-controlled piezoelectric oscillation circuit.
FIG. 33 shows an equivalent circuit which is equivalent to the voltage-controlled piezoelectric oscillation circuit during oscillation.
The voltage-controlled piezoelectric oscillation circuit is broadly divided into two portions: the piezoelectric resonator X and the remainder circuitry.
The piezoelectric resonator X can be expressed in terms of a series connection of an equivalent reactance L and an equivalent resistance R, while the remainder circuitry is expressed by a series connection of a load capacitance CL and a negative resistance -R.
FIG. 34 shows an equivalent circuit of the piezoelectric resonator.
The piezoelectric resonator X can be expressed in terms of a series connection which interconnects both terminals of the resonator X and which includes an equivalent resistance R1, an equivalent reactance L1 and an equivalent series capacitance CS, and an equivalent parallel capacitance CP which is in parallel with the above-mentioned series connection.
The following equation (1) expresses the relationship between the oscillation frequency fOSC and the load capacitance CL of the remainder circuitry in the voltage-controlled piezoelectric oscillation circuit implemented by using the piezoelectric resonator X. ##EQU1##
In this equation, the symbol .gamma. represents the capacitance ratio which is given as follows: EQU .gamma.=CP/CS
The symbol dfr is the frequency deviation, i.e., the deviation of the oscillation frequency fOSC from the series resonance frequency fr of the piezoelectric resonator X.
A description will now be given of the load capacitance CL.
FIG. 35 shows the connective relationship between the piezoelectric resonator X and various capacitances constituting the voltage-controlled piezoelectric oscillation circuit, as obtained when the varicap Cv and the trimmer CT are connected in series as in the first conventional arrangement.
Therefore, the load capacitance CL of the circuit in the first type of conventional arrangement is expressed by the following equation (2): ##EQU2##
In contrast, when the varicap Cv and the trimmer CT are connected in parallel--as in the second conventional arrangement, a connective relationship as shown in FIG. 36 exists between the piezoelectric resonator X and the capacitances constituting the voltage-controlled piezoelectric oscillation circuit.
Therefore, the load capacitance CL of the circuit in the second type of conventional arrangement is expressed by the following equation (3). ##EQU3##
[6] Comparison Between First and Second Conventional Arrangements
The first and second types of conventional arrangement will be compared with each other, on an assumption that the characteristics of the trimmer CT, varicap Cv, first oscillation capacitor CO1 and the second oscillation capacitor CO2 used in the first type of conventional arrangement are the same as those exhibited by corresponding components in the second type of conventional arrangement.
The first type of the of conventional arrangement shown in FIG. 31, in which the series connection of the varicap Cv and the trimmer CT is connected to one end of the piezoelectric resonator X, offers the following advantages over the second type of the conventional arrangement shown in FIG. 32, in which a parallel connection of the varicap Cv and the trimmer CT is connected to one end of the piezoelectric resonator X.
[6.1] Ease of Preservation of Adjustable Range of Oscillation Center Frequency f0 by the Trimmer
The first type of the of conventional arrangement shown in FIG. 31, in which the series connection of the varicap Cv and the trimmer CT is connected to one end of the piezoelectric resonator X, permits the trimmer to adjust the center frequency f0 over a wider range.
This makes it easier to adjust the center frequency f0 of the voltage-controlled piezoelectric oscillation circuit despite any variation in the characteristics of the piezoelectric resonator X. Consequently, the specifications for the production of the piezoelectric resonator X becomes less severe, contributing to reduction in the cost of production of the same.
[6.2] Ease of Preservation of Range over which the Frequency is Varied (f-Vc characteristic) by the Varicap.
The first type of conventional arrangement, in which the series connection of the varicap Cv and the trimmer CT is connected to one end of the piezoelectric resistor X in series thereto, permits easy preservation of the range over which the frequency is variable (fOSC-Vc characteristic) by the effect of the varicap.
More specifically, as will be seen from FIG. 38 showing the frequency deviation vs. Vc characteristic of the oscillation frequency fOSC, the series connection of the varicap Vc and the trimmer CT to one end of the piezoelectric resonator X permits a large change of deviation of the oscillation frequency fOSC, even with a small change in the control voltage Vc.
A description will now be given of the problems encountered by the described conventional arrangements.
[7.1] Restriction of the Range Over which fOSC-Vc Characteristic is Variable, Caused by Variation in Trimmer Capacitance
Application of the less severe specifications to the production of the piezoelectric resonators X require different trimmer capacitance values for different oscillation circuits, in order to adjust the oscillation center frequency in these circuits.
In the second type of conventional arrangement shown in FIG. 38 in which the parallel connection of the varicap Cv and the trimmer CT is connected to one end of the piezoelectric resonator X, a change in the control voltage Vc effected for the purpose of adjusting the oscillation center frequency f0 cannot cause a substantial change in the oscillation frequency fOSC, when the capacitance of the trimmer CT is set to 50 (pF) or 100 (pF). In such a case, it is not possible to obtain the desired fOSC-Vc characteristic.
In contrast, in the first type of conventional arrangement in which the varicap Cv and the trimmer CT are connected in series to one end of the piezoelectric resonator X, there is no risk of failure to obtain the desired fOSC-Vc characteristic, although the restriction in the range over which the oscillation frequency fOSC varies is inevitable.
It is thus possible to suppress the restriction or narrowing of the fOSC-Vc characteristic variable range caused by a variation in the trimmer capacitance, by adopting the first type of arrangement in which the series connection of the varicap Cv and the trimmer CT is connected to one end of the piezoelectric resonator X.
[7.2] Problems Attributable to Use of Trimmer
[7.2.1] First Problem
The number of parts or components to be incorporated in the piezoelectric oscillation circuit is preferably small, from the view point of production cost. If possible, all the parts and components are packaged on an IC chip so that both the assembly cost and parts cost are reduced.
Practically, however, it is difficult to integrate the piezoelectric resonator X, varicap Cv and the trimmer CI in a single IC chip, because of electrical characteristics and other requirements. Therefore, fabrication of a piezoelectric oscillation circuit essentially requires at least four discrete parts or components: namely, an IC incorporating the circuitry, the piezoelectric resonator X, the varicap Cv and the trimmer CT, despite the attempt to reduce the number of parts or components. This poses a practical limit in the reduction of the production cost.
[7.2.2] Second Problem
The trimmer CT is an electro-mechanical part having a mechanical rotary part for varying the capacitance. A practical limit therefore exists in the reduction of the size of the trimmer CT, from the view point of ease of manipulation.
Thus, the presence of the trimmer CT is one of the factors which cause impediment to miniaturization of piezoelectric oscillation circuit and saving of space.
[7.2.3] Third Problem
The trimmer CT, that has a mechanical rotary part for varying the capacitance, exhibits inferior stability for a long use, as compared with fixed-capacitance capacitors. In addition, the mechanical part tends to exhibit mechanical positional deviation when, for example, impacted by an external force, undesirably allowing deviation of the capacitance from the set value.
For these reasons, the use of a trimmer CT tends to impair the stability of the product piezoelectric oscillation circuit.
[7.3] Problem in Regard to Cost Incurred for Automatic Adjustment of Oscillation Center Frequency f0
FIG. 39 is a block diagram showing the configuration of an automatic adjusting system for adjusting oscillation center frequency f0 in the first type of conventional voltage-controlled piezoelectric oscillation circuit.
The automatic adjusting system 300 has an oscillation center frequency (f0) adjusting unit 301 which is connected to the output terminal OUT. The adjusting unit 301 is implemented by, for example, a personal computer that computes the amount of rotation of the rotary mechanism of the trimmer CT corresponding to the amount of capacitance adjustment performed by the trimmer CT and that outputs digital adjusting amount data based on the computed amount of rotation. The automatic adjusting system 300 also has a servo mechanism 302 that drives and rotate an adjusting screw serving as the rotary mechanism of the trimmer CT by an amount corresponding to the adjusting amount data.
The operation of the automatic adjusting system for adjusting the oscillation center frequency f0 is as follows.
The oscillation center frequency adjusting unit 301 of the automatic adjusting system 300 monitors the frequency of the oscillation signal SOSC which is output from the output terminal OUT, and supplies the servo mechanism 302 with adjusting amount data that corresponds to the amount of adjustment to be effected.
The servo mechanism 302 drives and rotates the adjusting screw of the trimmer CT to adjust the oscillation frequency to the desired target frequency.
It is not easy to accurately and quickly control the rotation amount of the, adjusting screw of the trimmer CT. Consequently, the yield or the number of product units of the voltage-controlled piezoelectric oscillation circuit is limited by the step of adjusting the oscillation center frequency f0.
This undesirably leads to a rise in the cost of production of the voltage-controlled piezoelectric oscillation circuit.